The present invention relates to a method for electrically connecting an electronic device such as IC chips and a circuit board. More particularly, the invention relates to a method for electrically connecting an electronic device to a glass-material circuit board, referred to as a glass substrate hereinafter, with high reliability and outstanding inexpensiveness and still in a high density of mounting per unit area of the circuit board by using electroconductive resinous bumps. The invention further relates to an electroconductive resinous composition used for forming the above mentioned bumps as well as a liquid-crystal display unit in which the electrical connection is obtained by the above mentioned method using the electroconductive resinous composition for forming the above mentioned bumps.
In the technology of plane image display such as dot-matrix liquid-crystal displays, dot-matrix plasma displays and the like in recent years, it is an outstanding trend that these display units are required to have a smaller and smaller thickness and weight while the surface area of the display plane is required to be larger and larger. These requirements are particularly pertinent to modern commercial products such as pocketable television sets, wall-mountable television sets, display units in personal computer systems, terminal display units in various kinds of communication instruments and the like.
One of the key technological points in these display units is in the electrical connection between the circuit board and the IC chips for driving the display unit. Namely, the circuit boards in these display units are mostly made with a glass substrate so that the conventional wire-bonding method is hardly applicable to the direct mounting of IC chips on the circuit board. Accordingly, several indirect methods have been proposed and practiced in many commercial products of the prior art for electrically connecting IC chips to a glass substrate on which the IC chips are mounted including a method in which an IC encapsulated in a flat package (FP) is mounted on a substrate formed of a glass cloth-epoxy resin laminate and the electrical connection with the circuit on the glass substrate is obtained by using a flexible flat connector (FFC) and a method in which the IC chip is mounted on a polyimide resin-based flexible substrate by the TAB method and both of the external terminals of the flexible substrate are connected to the circuit on the glass substrate and the printed circuit board of the control system. These indirect mounting methods, however, are disadvantageous because the IC chip must be mounted on a separate substrate other than the glass substrate so that an additional space therefor is necessitated while the volume of such an additional space must be exponentially increased along with the increase in the number of ICs for driving the display unit which is under the trend toward those having a larger and larger display screen.
Alternatively, several improved methods have been proposed for electrically connecting IC chips and a glass substrate by directly mounting the IC chips thereon. These methods are briefly described with reference to the accompanying drawing.
Firstly, as is illustrated in FIG. 3, an IC chip 21 is provided with bumps 22 of a solder alloy on the input and output electrodes and the IC chip 21 bearing the solder bumps 22 is mounted face-down on a metallized glass substrate 23 and the glass substrate 23 bearing the IC chip 21 is passed through an infrared-heating oven so that the solder bumps 22 are caused to reflow and the IC chip 21 is fixedly positioned by the reflow of the solder bumps 22 with electrical connection to the glass substrate 23. This method is called a flip-chip method.
Secondly, as is illustrated in FIG. 4, an IC chip 31 provided with gold material bumps 32 is mounted face-down on a glass substrate 33 coated beforehand with a pool of an ultraviolet-curable insulating resinous adhesive 34 all over the area for bonding corresponding to the dimension of the IC chip 31 including the input and output electrodes. Thereafter, the insulating adhesive 34 is irradiated with ultraviolet light while the IC chip 31 and the glass substrate 33 are gently pressed together so that the insulating adhesive 34 is cured in situ to fix the IC chip 31 at the position and to establish electrical connection thereof with the glass substrate 33. This method is called a gold bumps-adhesive combination method.
Thirdly, as is illustrated in FIG. 5, an anisotropically electroconductive rubber connector 43 made of an insulating sheet impregnated with anisotropically electroconductive rubber members at the respective proper positions is sandwiched between an IC chip 41 having bumps 42 and a glass substrate 44 and the IC chip 41 and the glass substrate 44 are fixed by pinching with a resilient holder clamp 45 to establish the electrical connection through the anisotropically electroconductive rubber connector 43. This method is called a rubber-connector press-contact method.
Fourthly, as is illustrated in FIG. 6, an IC chip 51 having gold-material electrodes 52 to serve as the input and output terminals is coated on the gold-material electrodes 52 by printing with a thermosetting or heat-curable electroconductive resin 53 and the IC chip 51 is mounted face-down on a glass substrate 55 having terminals thereof contacted with the terminals 52 of the IC chip 51 through the electroconductive resin 53 followed by heating of the assembly so that the heat-curable resin 53 is thermally cured to establish electrical connection between the terminals. This method is disclosed in Japanese Patent Kokai 53-59398. It is usual in this method that the gap between the IC chip 51 and the glass substrate 55 is filled with an epoxy resin 54 in order to enhance the adhesive bonding strength therebetween.
One of the essential requirements in the direct mounting methods of IC chips on a glass substrate is the repairability of the assemblage which means that any IC chip which is found unacceptable by the dynamic test of the assemblage can be easily replaced with another acceptable IC chip without damage to the circuit board and other parts. Namely, acceptability or unacceptability of IC chips can be determined only after the dynamic 100% inspection of the assemblages by using an autoprober to discard unacceptable ones while demounting of a single unacceptable IC chip may destroy the much more expensive glass substrate to cause a great economical loss. When such an unacceptable IC chip can be replaced with another acceptable IC chip without damage to the glass substrate, this economical loss can be greatly decreased so that the above mentioned repairability is a very important requirement.
In the above described gold bumps-adhesive combination method, for example, the adhesive once cured by forming crosslinks with ultraviolet irradiation can not be caused to lose its adhesive bonding strength so that the glass substrate is necessarily damaged when the unacceptable IC chip is forcibly separated therefrom.
Another essential requirement in the direct mounting methods of IC chips on a glass substrate is that the IC chips and the glass substrate are bonded together with high damping characteristics to withstand thermal shocks. It is noted that the linear thermal expansion coefficients of semiconductor silicon, which is the base material of IC chips, and glass, e.g., soda lime glass, which is the material of glass substrates, are 3.times.10.sup.-6 .degree.C.sup.-1 and 10.times.10.sup.-6 .degree.C.sup.-1, respectively, with a difference of 7.times.10.sup.-6 .degree.C.sup.-1. When an IC chip of 10 mm .times.10 mm dimension on a glass substrate is subjected to a heating-cooling cycles between +70.degree. C. and -20.degree. C., for example, the difference in the thermal expansion or thermal contraction between them amounts to about 7 .mu.m within the bonding plane which causes a considerable stress therein. In the above described flip-chip method in which the IC chip and the glass substrate are directly bonded together by melting the solder bumps, bonding and electrical connection between them are readily destroyed when such a thermal stress is repeated.
Still another essential requirement in the direct mounting methods of IC chips on a glass substrate is that the IC chips and glass substrate should be bonded together at low cost and with high reliability of the electrical connection and that the method be suitable for high-density mounting. Generally speaking, the methods by forming metal bumps are economically not advantageous in respect of the relatively low yield of acceptable products due to the complicated nature of the process. The rubber-connector press-contact method is disadvantageous because the anisotropically electroconductive rubber connector is relatively expensive due to the difficulties in the manufacture thereof and the method is not suitable for high-density mounting due to the bulkiness of the holder clamps. Further, the method using a heat-curable electroconductive adhesive has problems when the method is applied, for example, to liquid-crystal display units. Namely, the electroconductive adhesive must be cured by heating at a high temperature or for a long time so that not only may deterioration may take place in the liquid crystal per se but also several adverse influences are caused on other components of the liquid-crystal display unit such as shrinkage of the polarizing plate and denaturation of the color filter. Thus, each of the above described prior art methods is not satisfactory from a practical standpoint in one or more respects as a method for mounting an IC chip on a glass substrate.